1. Field of the Invention
The present invention involves the of a media support assembly in a printer, and more particularly the use of a self-centering media support assembly in a handheld printer.
2. Description of Related Art
Desktop printers typically require some type of a media supply (e.g., paper or labels) from which media is drawn and dispensed against a print head. The print head prints on the media (e.g., with ink, toner or heat) as it is passes against the print head. Eventually, after a certain amount of printing, the media supply is exhausted and needs to be replaced with a fresh media supply. Replacement of the media supply typically involves opening a housing of the desktop printer by unlocking one or more latching mechanisms, at which point a tray of the housing is released to allow access to the media supply of a media dispenser. The user can then refill or replace the media in the media dispenser, such as by placing a fresh stack of paper in the tray.
In handheld printers, size constraints typically dictate that the supply of printer media be more compact. For this purpose, rolls of media are often employed. Rolls of media typically need to be positioned with respect to the print head. U.S. Pat. No. 6,609,844 (“the '844 patent), for example, discloses a portable printer 10 having an automatic print alignment. The portable printer includes a housing 12 with an upper housing section 12a which mates with a lower housing section 12b, and a cover 14 for a compartment 16 in the printer which receives a roll of paper or label stock, as shown in FIGS. 1 and 2 of the '844 patent. The roll 15 is made of thermally sensitive paper or label stock that is pulled upwards by a platen roller 24 over a print head 33 as it prints on the paper or label stock.
Placement and replacement of the roll is facilitated by a centering mechanism 36 that ensures that each new roll is centered with regard to the print head, regardless of the width of the roll, as shown in FIGS. 3 and 4 of the '844 patent. The centering mechanism includes two spindles 38 in the compartment for engaging the roll's tubular core and a rack and pinion assembly that enables movement of the spindles with respect to a center position. The rack and pinion assembly includes two racks 40a and 40b each with teeth engaging the teeth 42a of a common pinion or gear 42 and each supporting one of the spindles. An extension spring 56 has one end attached to rack 40a and applies tension on the rack 40a directly and on the rack 40b through the pinion, thereby drawing the spindles together to hold a roll of printer media when placed between the spindles.
A locking mechanism is provided to lock the centering mechanism from substantial movement when the cover is latched closed by a latch member 68. A rack lock 86 of the mechanism has a cylindrical shape with an open end 86a and projections 87 for engaging additional pinion teeth 54. The rack lock is mounted on a plate 90 having two notches 92 mounted to slide along two tracks 93 extending downwardly from a frame 30 of the printer. This positioning allows the rack lock to engage the pinion teeth with only a slight forward movement.
A compression spring 96 biases the rack lock away from the pinion teeth. However, the bias of the compression spring can be overcome by movement of the latch member to the closed position which depresses a lever 110 and turns a shaft 102 supporting the lever. Turning of the shaft pushes the rack lock forward against the bias of the compression spring until the rack lock engages the pinion, halting movement of the pinion, the racks and the spindle members. Latching the cover of the printer, therefore, locks the media roll in a centered position with respect to the print head to facilitate printing. Although the printer of the '844 patent advantageously centers and locks a media roll in place for a hand held printer, additional improvements in the compactness and simplicity of the operation of hand held printers is always desirable. In particular, the complexity of the centering mechanism detracts from its impact resistance. For instance, during drop tests complex centering mechanisms have a tendency to stop functioning due to the centering mechanisms “jumping track,” experiencing tooth failures or skipping cogs.
Therefore, it would be advantageous to have a printer that has a simplified mechanism for centering and holding media. Also, it would be advantageous to have a centering mechanism that is fairly robust and resistant to drops and other impacts. It would be further advantageous if the simplified mechanism were also compact so as to facilitate its use in a hand held printer.